Mass flow meters for gases measure the mass flow rate of a gas independently of gas temperature or pressure. Forms of such devices which operate on heat transfer principles have become widely adopted. A common commercial form incorporates a small diameter tube which has two coils of wire wound on the outside in close proximity to each other. The coils are formed from a metallic material having a resistance which is temperature-sensitive.
In a bridge-type electrical circuit, the coils can then be heated by an electrical current to provide equal resistances in the absence of flow of the gas and a balanced condition for the bridge-type circuit--e.g., a null output signal.
Then, with the gas flowing within the tube, within the relevant measuring range of the device, the temperature of the upstream coil is decreased by the cooling effect of the gas and the temperature of the downstream coil is increased by the heat from the upstream coil transmitted by the fluid. This difference in temperature is proportional to the number of molecules per unit time flowing through the tube. Therefore, based on the known variation of resistance of the coils with temperature, the output signal of the bridge circuit provides a measure of the gas mass flow.
In various circumstances, forms of heat transfer phenomena can introduce substantial error in the measurements of these mass flow meter devices. U.S Pat. No 3,938,384, issued Feb. 17, 1976, and U.S. Pat. No. 4,056,975, issued Nov. 8, 1977, both having the same Assignee as herein, are illustrative of the problem.
As discussed in the latter of these patents, at relatively elevated pressure levels of the gas, the error introduced by free convection of the gas within the tube becomes relatively dominant. The result, for such higher pressure levels, is a substantial error due to such convection when the device is tilted with respect to the direction of gravity. As discussed in both of these patents, at relatively lower pressures, the effects of this sort of convection are not substantial; however, the error introduced by free convection by the ambient gas outside the tube becomes a dominant source of error with variations in the attitude of the device with respect to gravity. In U.S. Pat. No. 3,938,384, the first-mentioned patent, this sort of convective effect is addressed by encapsulating the tube with the coils thereabout, in the vicinity of the coils, with an open cell foam material. Although, as indicated in the patent, the advantages of that approach are substantial, it does bring certain detriments. First, it slows the response of the device as a result of the presence of the foam material. Second, the calibration of the device can shift with time as the foam changes its chemical composition or its degree of contact with the coils and conduit. Third, it reduces the gain of the device.
A general approach to the convection outside the conduit, of which the just-mentioned approach may be considered a specific form, involves the use of various materials to contact the coils in order to keep convective currents from transferring heat externally from one coil to the other. This general approach typically is unsatisfactory for a variety of reasons, the most important one usually being the reduction of the level of response of the device to changes in flow.
Yet more generally, flow meter devices such as those discussed above, are commonly enclosed in some type of container to isolate their sensitive parts from outside air currents and outside localized sources of heating or cooling. This, of course, is a distinct concern from the effects of convection immediately adjacent to such sensitive parts.
Of some, related interest are some propositions that have been put forward with regard to heat transfer phenomena in gases. As far back as 1912, I. Langmuir, in "Convection and Conduction of Heat In Gases", The Physical Review, Vol. 34, No. 6, June 1912, pp. 401-422, proposed that there is a thickness for a layer of gas on a plane for which loss of heat from the plane through conduction strongly dominates over loss through convection, and that this thickness is a constant independent of the temperature of the plane (at least at a given temperature and pressure for the surrounding environment). For air, such thickness "B", at about room temperature and pressure, is proposed to be about 0.43 cm (about 0.17 inch). The analogous situation, then, is said to apply to a layer of the gas of outside diameter "b" about a wire of outside diameter "a", wherein, b ln (b/a)=2 B.
The present invention addresses long-standing problems and concerns with attitude sensitivity in gas mass flow meters stemming from convective heat transfer outside a tube through which the gas is directed. It does so while also addressing the goals of high sensitivity and rapid responsiveness to changes in flow rate.